The Global System for Mobile (GSM) telecommunications is used in cellular telephone networks in many countries around the world. GSM offers a useful range of network services and standards. Existing GSM networks are based on time-division multiple access (TDMA) digital communications technology. In a TDMA-based cellular network, each mobile subscriber unit communicates with only a single base station at any given time. When a subscriber moves from one cell to another, a “hard handover” takes place, in which the base station with which the subscriber has been communicating breaks off its link with the subscriber, and a new base station takes over.
Code-division multiple access (CDMA) is an improved digital communications technology, which affords more efficient use of radio bandwidth than TDMA, as well as a more reliable, fade-free link between cellular telephone subscribers and base stations. The leading CDMA standard is IS-95, promulgated by the Telecommunications Industry Association (TIA).
This standard provides “soft handover” (or “handoff”) capability, wherein in moving from one cell to another, the subscriber unit is temporarily in contact with two or more base stations at the same time. This soft handover, which is made possible by the code-division approach, decreases the likelihood of a loss of connection, which can happen frequently in hard handovers.
PCT patent application PCT/US96/20764, which is incorporated herein by reference, describes a wireless telecommunications system that uses a CDMA air interface (i.e., basic RF communications protocols) to implement GSM network services and protocols. Using this system, at least some of the TDMA base stations (BSSs) and subscriber units of an existing GSM network would be replaced or supplemented by corresponding CDMA equipment. CDMA BSSs in this system are adapted to communicate with GSM mobile switching centers (MSCs) via a standard GSM A-interface. The core of GSM network services is thus maintained, and the changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and CDMA elements, are also described in PCT patent publications WO 95/24771 and WO 96/21999, and in an article by Tscha, et al., entitled “A Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching—Center,” in Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa (1993), pp. 181-185, which are incorporated herein by reference. None of these publications deals with specific issues of how to perform efficient handovers of subscriber units between different base stations in such hybrid networks.
PCT patent application PCT/US97/00926, which is also incorporated herein by reference, describes methods of intersystem handover between CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunications system. A GSM/TDMA BSS generates pilot beacon signals in accordance with CDMA technology. During a telephone call, a subscriber unit detects the pilot signals and notifies a base station controller that the signals have been detected. The subscriber unit is then handed over from the CDMA to the TDMA BSS without interrupting the call.
The International Telecommunications Union recently requested the submission of proposed methods for providing high-rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission,” and hereinafter referred to as cdma2000. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission,” also known as “wideband CDMA,” and hereinafter referred to as W-CDMA. A third proposal was submitted by U.S. TG 8/1, entitled “The UWC-136 Candidate Submission,” hereinafter referred to as EDGE. The contents of these submissions are public record and are well known in the art.
Two known radio-frequency (RF) interfaces for such so-called “third generation” wireless communication systems are the multicarrier (MC) over-the-air interface and the direct spread (DS) over-the-air interface. A third generation system employing the MC air interface may be a system that uses network signaling protocols specified by American National Standards Institute (ANSI) 41. The proposed cdma2000 system is such a system. In the alternative, a system using an MC air interface may employ network signaling protocols defined by the Mobile Application Part (MAP) of the GSM-MAP standard described above. Likewise, a system may employ a DS air interface and ANSI 41 network signaling protocols, or a DS air interface and MAP network signaling protocols. The proposed WCDMA system uses the DS air interface and MAP network signaling.
Just as for GSM and CDMA systems, intersystem handover is also necessary in areas where MC system coverage (e.g., cdma2000 base stations) gives way to DS system coverage (e.g., WCDMA base stations), or vice versa. It is also necessary to provide efficient timing synchronization between base stations of the two systems in that are communicating with a mobile unit during the handover.